1. Field of Invention
The present invention relates generally to semiconductor processing equipment. More particularly, the present invention relates to a method and an apparatus for enabling a Z-actuator and an anti-gravity device to be substantially combined in a single device.
2. Description of the Related Art
For precision instruments such as photolithography machines which are used in semiconductor processing, factors which affect the performance, e.g., accuracy, of the precision instrument generally must be dealt with and, insofar as possible, eliminated. By way of example, excessive vibration often adversely affects the performance of precision instruments. When the performance of a precision instrument such as a wafer table is adversely affected, products formed using the precision instrument may be improperly formed and, hence, function improperly.
Many wafer tables which are used to support and to scan wafers are typically able to move in a z-direction, i.e., translate along a z-axis, and also rotate about an x-axis and about a y-axis. The ability for a wafer table to move in a z-direction, as well as to rotate about an x-axis and a y-axis, allows for focus and leveling control of the wafer table. The weight of such wafer tables, i.e., wafer tables which may translate relative to a z-axis as well as rotate relative to at least one of an x-axis and a y-axis, typically needs to be supported against the force of gravity.
In addition, a wafer table generally needs to be guided in order to control translational motion along the x-axis and the y-axis, as well as to control rotational motion about the z-axis, during focusing and leveling control. That is, while a wafer table is translating relative to the z-axis, and rotating about the x-axis or the y-axis, the wafer table typically needs to be prevented from rotating about the z-axis, and translating along the x-axis and the y-axis.
To enable a wafer table to be supported against the force of gravity and guided to reduce unwanted motion during focusing and leveling control, multiple voice coil motors (VCMs) may be used to effectively actuate the wafer table while a permanent magnet supports the weight of the wafer table. One wafer table that uses three VCMs for actuation, and has its weight supported by a permanent magnet, is described in U.S. Pat. No. 6,040,675 issued to Kazuya Ono, entitled “Supporting Apparatus Using Magnetic Power,” which is incorporated herein by reference in its entirety. As described in U.S. Pat. No. 6,040,675, three VCMs are used to actuate a wafer table, while a permanent magnet is used to support the weight of the wafer table and a flexural bearing is used to guide the table.
While the use of multiple VCMs, e.g., three VCMs, a permanent magnet, and a flexural bearing is generally adequate to achieve desired results during focusing and leveling control of a wafer table, such a wafer table has a relatively high stiffness in a Z-direction. This relatively high stiffness in the Z-direction typically leads to relatively poor vibration transmissibility. Poor vibration transmissibility may compromise the integrity of a wafer that undergoes an exposure process while supported on the wafer table. Further, a flexural bearing may not be suitable for use if a wafer table is to utilize more than three degrees of freedom. For example, a six degree of freedom wafer table would not allow for the use of a flexural bearing, since a six degree of freedom wafer table may not be constrained in a direction that is typically constrained by a flexural bearing.
Therefore, what is needed is a method and an apparatus that allows a wafer table to have translational motion along a z-axis, rotational motion about an x-axis, and rotational motion about a y-axis while supported against the force of gravity and guided to reduce unwanted motion. That is, what is desired is a method and an apparatus for allowing focus and leveling control to occur while a wafer table is guided and supported against the force of gravity, without limiting the number of degrees of freedom associated with the wafer table.